In wells for collecting petroleum and natural gas buried resources, use of a drilling mud and a drilling cement slurry has been conventionally known. The functions of the drilling mud may involve: transporting drilled clasts and drilling wastes; lubricating bits and drill pipes; filling in holes on the porous ground; and balancing out the reservoir pressure that results from the hydrostatic pressure. Such a drilling mud is required to have a great specific gravity, which may be attained by adding preferably, barite, salt or clay. In addition, the drilling mud is also required to have temperature stability, and appropriate flow characteristics that are not significantly affected by variation of concentrations of electrolytes. In order to achieve these performances, adjusting the viscosity of the drilling mud, and inhibiting dissipation of the moisture contained in the drilling mud (hereinafter, may be also referred to as “dehydration”) may be an exemplary measure. Thus, a method which includes adding a polymer, specifically, for example, starch, a starch ether (carboxymethyl starch, etc.) carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose or the like as an additive is usually adopted. However, these additives may extremely elevate the viscosity of the drilling mud, whereby injection by the pump may be difficult. Moreover, at a temperature of higher than about 120° C. (in the cases of a starch and derivatives thereof) or at a temperature of 140 to 150° C. (in the cases of carboxymethyl cellulose and carboxymethyl hydroxyethyl cellulose) it may be inconvenient that inhibition of dehydration may not be sufficient.
On the other hand, the drilling cement slurry has been used for filling in tubular void portions between the stratum and the casing pipe installed in the well with a cement material to provide hardened corresponding portions. In general, the drilling cement slurry is fed into tubular void portions between the stratum and the casing pipe with a pump, and then the tubular void portions are sequentially filled with the drilling cement slurry from the well bottom, followed by hardening. Thus, the casing pipe is fixed in the well, thereby protecting the inside wall of the well. The drilling cement slurry is required to be readily injected by a pump. In other words, the drilling cement slurry is required to have extremely low viscosity, and not to be accompanied by separation.
Conventionally employed drilling cement slurries for use in cementing wells contain various types of additives. These additives are exemplified by a cement accelerator, a cement retarder, a dispersant, a cement dehydration-reducing agent, a low-density additive, high-density additive, an cement-expanding agent, a cement strength stabilizer and a silica powder and the like which have been used in combination depending on the well conditions and purposes.
In cementing a well, a defect is likely to occur in a cemented part by material segregation, running into cracks in the well, and the like. To address such a defect, walnut shells, cotton seeds, clay minerals, polymer compounds and the like have conventionally been added. In particular, vinyl alcohol polymers are a well-known dehydration-reducing agent. Patent Document 1 discloses a method in which a vinyl alcohol polymer having a degree of saponification of at least 95 mol % is used, and Patent Document 2 discloses a method in which a vinyl alcohol polymer having a degree of saponification of 92 mol % or less is used. However, also according to these methods, dehydration-reducing performances at high temperatures in particular, may be insufficient, or feeding of the drilling cement slurry by the pump may be difficult through elevating the viscosity.
In order to improve the dehydration-reducing performances, and/or to moderate the viscosity elevation of the drilling cement slurry at high temperatures: Patent Document 3 discloses use of a vinyl alcohol polymer crosslinked by melamine-formaldehyde; Patent Document 4 discloses use of a vinyl alcohol polymer produced by crosslinking of hydroxyl groups of a vinyl alcohol polymer by an acetalization reaction or the like; and Patent Document 5 discloses use of a vinyl alcohol polymer produced by pH-sensitive crosslinking using a boron ion, etc. Although a given effect of improving the dehydration-reducing performances at high temperatures was achieved in every case, it is necessary to carry out the reaction with the crosslinking agent after a water soluble vinyl alcohol polymer was produced beforehand, leading to an economical drawback.